Fix direct imports to prose styles

[aduth]

🛠 Summary of changes

Regression of #390

Fixes an issue where code directly importing usa-prose would fail due to missing _overrides.scss file removed in #390.

Exception [Error]: Error: Can't find stylesheet to import.
  ╷
2 │ @forward './overrides';
  │ ^^^^^^^^^^^^^^^^^^^^^^

The project's current build pipeline would only catch errors occurring in a full build. TBD I need to more fully understand how this doesn't cause a failure within this project, but the short of it seems to be that this file isn't being loaded.

Because we no longer customize the component as of #390, we don't need these files, since the design system is already set up to allow "pass through" to USWDS for uncustomized components.

📜 Testing Plan

I reproduced the issue on main by testing in identity-idp.

In identity-idp:

1. Change @18f/identity-design-system dependency in package.json to point to file:../identity-design-system
2. rm -rf node_modules
3. yarn
4. yarn buld:css

Before: Fails with error mentioned above
After: Succeeds

[aduth]

Visual regression is expected since removal of overrides file causes "View Login.gov SCSS on GitHub" link to be removed from internal documentation.